Several publications and patent documents are cited throughout the specification in order to describe the state of the art to which this invention pertains. Each of these citations is incorporated herein by reference as though set forth in full.
In response to vascular injury such as a cut, coagulation enzymes are activated in a stepwise manner, ultimately resulting in the formation of a blood clot at the site of injury. Thrombin is generated from its inactive precursor prothrombin in the final step of this cascade and subsequently produces the fibrous clot. Activated Factor IX (FIXa) is a key component of this system as it is the serine protease of the intrinsic Xase complex, which also comprises the co-factor activated Factor VIII (FVIIIa). This enzyme, assembled on cells with exposed anionic membranes, rapidly converts Factor X to activated Factor X (FXa). FXa and its co-factor, activated Factor V (FVa), form prothrombinase, the enzyme complex that activates thrombin.
The importance of Factor IX is reflected by the occurrence of the bleeding disorder hemophilia B in individuals carrying mutations in the Factor IX gene. In bleeding disorders such as hemophilia B, a defect or deficiency of Factor IX results in inadequate FXa generation and, therefore, inadequate thrombin formation. Replacement of the missing protein is the mainstay of hemophilia treatment. Although effective, there are several limitations with current protein replacement therapies. For example, current therapies suffer from the short-half life of the proteins administered, thereby requiring multiple intravenous injections at high doses. Therefore, there is an obvious need for clotting factors with improved biological properties.